Change size of header logo

Can anyone tell me how to change the size of the header logo from 280px to 960px.

I have created a child theme, changed the CSS to:

#headimg #logo-image {
width: 960px;
overflow: hidden;
}

#headimg #logo-image img {
max-width: 960px;
height: auto;
width: 100%;
border: none;
}

I removed the search box from displaying in header.php. The theme option to upload a header crops and reduces the image size automatically to 280px so with the CSS set to 960px my header logo displays as a very stretched (it has used the image it cropped to 280px during the upload header process and stretched it to 960px)

Thanks very much